Linear motion guide units have been extensively applied in recent years between tow parts which slide relative to each other for linear reciprocating mechanism used in increased industrial fields including semiconductor fabricating equipment, machine tools, industrial robots, and so on. Most prior linear motion guide units have been ordinarily fed with lubricant every a preselected interval through any oiling port to ensure smooth lubrication for the rolling elements throughout a closed or looped circuit. Recently advanced machines and instruments, however, are increasingly wanted energy saving, simple construction and less production cost, and further in need of making them virtually maintenance-free from many aspects of keeping running cost and maintenance cost of equipment reasonably less than ever. Correspondingly, the linear motion guide units incorporated in the advanced machinery are also challenged to keep down the consumption of lubricant to a minimum, along with realizing virtual maintenance-free operation where lubricant resupply is less needed over long-lasting operation.
In Japanese Laid-Open Patent Application No. 2003-090338 which is a commonly-assigned senior application, there is described an example of the linear motion guide unit in which a turnaround passage is constructed to help smooth running of the rolling elements and application of lubricant around the rolling elements is done at the turnaround passage over a prolonged period to make sure of improved durability. The prior linear motion guide unit, as shown in FIG. 9, is constituted with a guide rail 1P having raceway grooves 4P on longitudinal sides thereof, one to each side, and a slider 2P which fits over or conforms to the guide rail 1P to move back and forth along the guide rail 1P. The slider 2P is composed of a carriage 10P having raceway grooves 14P cut to extend in opposition to the raceway grooves 4P on the guide rail 1P, end caps 11P fastened to lengthwise opposite ends of the carriage 10P, one to each end, and end seals 13P secured on outside surfaces of the end caps 11P. A plurality of balls 20P serving as rolling element is built in a closed or looped circuit made up of a load-carrying raceway 21P defined between the raceway grooves 4P and 14P, turnaround passages provided in the end caps 11P and a return passage 22P made in the carriage 10P. The return passage 22P in the carriage 10P is constituted with a lengthwise through-hole defined inside a return passage member 6P of sintered resinous material of porous texture, which fits into a fore-and-aft bore 5P made in the carriage 10P. The return passage 22P defined inside the return passage member 6P is connected with the turnaround passage through a spigot joint formed in the end cap 11P.
Another Japanese Laid-Open Patent Application No. H08-130849, now matured into JP patent No. 3 174 232, discloses an actuator comprised of a guide track having a pair of longitudinal side walls spaced sidewise away from each other and a slider that fits into the guide track to move inside the guide track in longitudinal direction. With the prior actuator, the slider has a pair of raceway grooves lying sidewise spaced away from each other to provide load-carrying races to allow rolling elements running through there. The slider has another pair of deep grooves lying between the raceway grooves extending lengthwise in parallel with the raceway grooves. Tubular members loosely fit into the deep grooves to leave small clearances around the tubular members. The tubular members each are made of seamless tubes of wear proof stainless steel and has an overall length beyond the deep groove by a preselected dimension. The tubular members are kept in place by engagement at their opposite ends thereof with a pair of sockets on end caps or return caps.
With the linear motion guide unit disclosed in the former patent literature, meanwhile, the return passage member 6P to define the return passage 22P in the carriage 10P of the slider 2P, as shown in FIG. 9, is connected to the turnaround passage through the spigot joint to make sure of smooth transfer of the rolling elements between the turnaround passage and the return passage. The return passage member 6P is made of a sintered resinous material of porous or continuous-cellular texture to make certain of steady and sustainable application of lubricant around balls 20P over a long-lasting period with improving durability. However, the prior construction in which the return passage 22P was buried or inserted into the carriage 10P of the slider 2P had need of drilling or boring the lengthwise through-hole 5P in the carriage 10P and further inserting carefully the return passage member 6P into the through-hole 5P to finish the return passage 22P. This means the drilling or boring operation to the carriage 10P wants high precision with difficulties and high working cost.
With the actuator disclosed in the latter patent literature, moreover, the guide track has a pair of longitudinal side walls spaced sidewise away from each other and the slider fits into the guide track to move inside the guide track. The slider has a pair of deep grooves underneath to accommodate the tubular members therein, one to each recess. The end caps have sockets which are made somewhat larger than the tubular members to fit snugly over the ends of the tubular members to finish the opposite ends of the return passages. With the actuator having the slider in which the tubular members are installed as stated in the latter patent literature to prepare the closed or looped circuit for the rolling elements, namely, it was necessary to bore or deep cut the slider to install the tubular members into the slider. This involves a problem of high working costs. Moreover, the joint construction that the tubular members fit into the end caps is apt to cause a specific difficulty in which any gap or difference might come at mating surfaces between the tubular members and the end caps. Meanwhile, the rolling elements turning around in the end caps in general experience a large centrifugal force or inertial force when especially transferring from a curved area in the turnaround passage to a straight area in the closed circuit. This poses a serious challenge that the rolling elements get caught or huddled together to render the sliding performance worse. In addition, other challenges might be caused in which the rolling elements after having hit against the gap or difference raised excessive wear on either of the tubular members and the end caps, and the rolling elements after having rolled with staggers huddled together, thereby worsening the sliding condition of the slider.